> My Apache modulator squeals when the microphone gain is raised > and the modulator tubes draw a lot of current. As long as the > microphone gain is kept very LOW the current does not take off > and there is no squeal coming from the modulator section. I am > going to go through the section with a screwdriver and a > nut driver to tighten up the grounds to the chassis and tube > socket.
In my experience the most common cause of these symptoms isn't feedback within the modulator itself, but RF rectification somewhere in the audio chain prior to the gain pot. Generally this is an indication of too much RF in the shack. I had this problem when I was using balanced line and a non-optimal tuner for this situation, creating imbalance in the system and subsequent radiation from the feedline. I have since built myself a proper balanced tuner at that operating position which has eliminated the problem. You can try wrapping the microphone cable around a ferrite or powdered iron core of the type used for RF filtering. Usually 3-4 turns will take care of it if it's going to at all. Put the core as close to the mike jack at the transmitter as you can. Trying a different style of microphone can also help. If you are running an EQ and/or processing ahead of the microphone input these devices can also be susceptible to RF rectification. 73/GL, -Larry/NE1S ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
